1. Field of the Invention
This invention relates to a shield tunnelling apparatus provided with a target for receiving a light ray directed along the axis of a shield body.
2. Description of the Prior Art
In some pipe propulsion engineering methods, a shield tunnelling apparatus and a main thrusting apparatus for advancing the tunnelling apparatus are used together with pipes following the tunnelling apparatus. When the direction of advance (advance direction) of the tunnelling apparatus deviates from a planned line of a tunnel to be constructed, directional correction is carried out.
This kind of directional correction is generally done using a light ray such as a laser beam directed along the axis of the tunnel to be constructed as a reference line. A target for receiving the light ray is disposed in the tunnelling apparatus. The advance direction of the tunnelling apparatus is then corrected such that the light ray is incident on a predetermined portion of the target.
Since the light ray acts as the reference line, the travelling direction of the light ray is along the axis (planned reference line) of the tunnel to be constructed, and the target is disposed such that the center of the target is aligned with the axis of the tunnelling apparatus.
When the axis of the tunnelling apparatus is aligned with the planned reference line, the light ray travels along the axis of the tunnelling apparatus and is incident on the center of the target.
On the other hand, when the axis of the tunnelling apparatus deviates from the planned reference line, the light ray is not incident on the center of the target. In this case, the advance direction of the tunnelling apparatus is corrected such that the light ray is incident on the center of the target.
For a tunnel having a bore which is too small to allow an operator to enter, the installations disposed in the shield body of the tunnelling apparatus hinder the light ray from travelling along the axis of the tunnel, however. Particularly when the excavation distance is long, an auxiliary pump for draining excavated matter should be disposed in the tunnelling apparatus. This auxiliary pump blocks the light ray, however. In tunnelling apparatus in which a light ray is directed along the axis of the tunnel, the size and amount of installations that may be disposed in the shield body are limited.
To solve the installation limitation problem, a light ray directed away from the axis of the tunnel is used. In this case, however, a reference line for measurement deviates from the axis of the tunnel, and the directional correction should therefore be conducted considering the displacement of the light ray from the axis of the tunnel. As a result, the directional correcting operation becomes troublesome.